1. Technical Field
The present invention relates to coherent optical detection and, in particular, to methods and systems for frequency offset estimation in coherent detection that is effective over a wide range.
2. Description of the Related Art
With the recent advance of high-speed analog-to-digital converters (ADC) and digital signal processing circuits, coherent detection has attracted strong interest because such a scheme, in conjunction with advanced modulation formats, can offer higher spectrum-efficiency and better receiver sensitivity over direct detection. In coherent receivers, received electric field information can be retained to allow digital signal processing (DSP) techniques to cope with the transmission impairments of a system. One key DSP function is to recover the carrier phase using DSP-based phase estimation (PE) rather than optical phase-locked loops, thus allowing for a free-running local oscillator (LO) laser. Some popular phase estimation algorithms require that the frequency offset between transmitter and LO laser be quite small compared to symbol rate, such that both lasers operate at nearly the same frequency. These PE algorithms fail to work when the frequency offset is larger than 1 GHz in a 10 Gsymbol/s coherent system using dual-polarization quadrature phase shift keying (QPSK) modulation format, resulting in a failure to match frequencies beyond this range. Commercial tunable lasers, however, have end-of-life frequency accuracy of about ±2.5 GHz. In other words, the frequency offset between transmitter and LO lasers can be as large as ±5 GHz, resulting in equipment failure when the frequency offset exceeds the relatively small range of conventional PE algorithms.